Can we create giveaways for unknown games?

No.

Not with the system of Steamgifts. But you can still make a normal forum thread, explain the situation and gift it away there. You won't get any "benefits" from it, though, if that is what you are after. But just giving away something is easy enough.

If you've a general idea of what the key is, you can try activating it, or have someone [or some bot] with a large library try and activate it. If the individual activating it already owns the game, Steam'll pop up an install window for the game (if they don't already have it installed) listing the name of the game. Naturally, you run the risk of actually activating the game to the account in question, if the account doesn't already own that game.

If you're not able to determine the title of the game, it's not permissible to offer it through Steamgift's core giveaway system, as that system requires giveaway creators to deliver the game listed. You could of course petition cg to implement a 0CV "Mystery Game" listing into the giveaway list. However, it's not too likely that cg'd agree to such a request, given that the risk of users already owning the game, and the additional demands that would place on our already overwhelmed site staff.

As Adelion indicated, you're probably going to have to just decide between activating it yourself or offering it to someone else to activate. If you're wanting some degree of recompense, you may see if someone is willing to gamble a trade on it against some Steam trading cards.

Okay can't create giveaway, can post in orphan key threads. Got it. Thank you.